1. Technical Field
The present invention relates to a wireless transmission systems in which a plurality of wireless stations in proximity to each other send data by multi-station simultaneous transmission, and a wireless station and method for use in the same.
2. Background Art
Generally, in wireless communication, a transmitted signal propagates through a plurality of propagation paths to reach a receiver in different propagation times, which causes multipath fading. To prevent a deterioration in transmission characteristics due to multipath fading, modulation/demodulation schemes resistant to multipath have been used.
Examples of the multipath resistant modulation/demodulation scheme include spread spectrum, Orthogonal Frequency Division Multiplexing (OFDM) in which information is distributed over a large number of subcarriers arranged within a wide range of frequencies, anti-multipath modulation in which multipath resistance is achieved by adding phase or amplitude redundancy to transmitted symbols, and the like.
Examples of spread spectrum include Direct Sequence Spread Spectrum (DSSS) in which an original signal is multiplied by a spread signal having a wider band than that of the original signal, Frequency Hopping Spread Spectrum (FHSS) in which a carrier signal is caused to hop over a wide band of frequencies, and Time Hopping Spread Spectrum (THSS) in which a signal is spread using impulses within a wide band.
Examples of the anti-multipath modulation scheme include PSK-VP (Phase Shift Keying with Varied Phase) in which convex-shape phase redundancy is added (Non-Patent Document 1), PSK-RZ (Return to Zero Phase Shift Keying) in which amplitude redundancy is added (Non-Patent Document 2), and the like.
Also, even when a typical single carrier modulation scheme is employed to perform wireless communication, multipath resistance can be imparted by providing an equalizer on the receiver's side. A modulation/demodulation scheme in which a single carrier modulation scheme is employed for wireless communication and an equalizer is used on the receiver's side, is also a multipath resistant modulation/demodulation scheme.
By using such a multipath resistant modulation/demodulation scheme for communication, a deterioration in transmission characteristics due to multipath waveform distortion can be prevented. Further, when element waves forming multipath (delayed waves) have moderate differences between their arrival times to a receiver, delayed wave components may be separated or combined by performing diversity reception (path diversity reception) with respect to the delayed waves, so that transmission characteristics can be actively improved.
Hereinafter, the lower limit value and the upper limit value of the arrival time difference that allows the path diversity effect are referred to as a delay resolution and a maximum delay, respectively. The delay resolution and the maximum delay may be determined by the principle of a modulation/demodulation scheme used or by parameters, or constraints on implementation, of the modulation/demodulation scheme.
For example, in the DSSS scheme, a received signal is separated into a plurality of delayed wave components (paths), which are in turn combined (RAKE reception), on the receiver's side. The delay resolution for obtaining the path diversity effect corresponds to the one-chip length of spread code. The maximum delay corresponds to a value that is less than the spread code length.
Also, in the OFDM scheme, a delayed wave component is absorbed in a guard interval set for a signal, and the maximum delay corresponds to a guard time. If a difference in propagation time between delayed waves is within the guard interval, symbol-to-symbol interference does not occur. Also, since an error correction process is typically performed over a plurality of subcarriers, information can be reproduced even if an error occurs in a portion of the subcarriers due to multipath distortion. On the other hand, the delay resolution is substantially equal to about the reciprocal of a frequency bandwidth. Thus, when the OFDM scheme is used, the path diversity effect can be obtained by the effect of the guard interval and the frequency diversity effect by distributing information over a wide frequency band and recovering the information.
Also, when the PSK-VP scheme or the PSK-RZ scheme is used, the delay resolution is equal to a time of a fraction of a symbol length, and the maximum delay is equal to a time of less than a one-symbol length. Also, when a single carrier scheme, such as the PSK scheme, the QAM scheme or the like, is used on the transmitter's side, and an equalizer using a delay line with a tap is used on the receiver's side, the delay resolution is equal to a one-symbol length, and the maximum delay is equal to a time determined by the number of taps.
In the fields of cellular telephone or broadcasting, a wireless transmission system has been proposed in which, by using the above-described anti-multipath modulation/demodulation scheme, when the same signal is sent by multi-station simultaneous transmission from antennas of a plurality of base stations, the signal is artificially delayed to obtain the path diversity effect, whereby transmission characteristics are actively improved, a communication area is increased, and the like. Note that, even in such a multi-station system, the path diversity effect cannot be obtained at a point where the arrival time difference of arriving waves from antennas departs from the range of the delay resolution or more and the maximum delay or less.
In fact, for example, when the arrival time difference of arriving waves from two stations is extremely small, the signals cancel each other at a point where two delayed waves having equal power and opposite phases are simultaneously received, so that transmission characteristics are significantly deteriorated. On the other hand, also at a point where the arrival time difference of arriving waves from two stations exceeds the maximum delay, the path diversity effect is not obtained, and in addition, the transmission characteristics are deteriorated. Therefore, in a conventional multi-station system, to avoid such a problem, an appropriate difference is provided between transmission timings at which a plurality of antennas perform multi-station simultaneous transmission, thereby making it possible to reliably obtain the path diversity effect (e.g., Patent Document 1).
FIG. 48A is a diagram showing a configuration of a conventional multi-station simultaneous transmission system described in Patent Document 1. In FIG. 48A, a base station 50 communicates with mobile terminals using the CDMA (Code Division Multiple Access) scheme. Remote antenna systems 52-1 to 52-n are located between the base station and mobile terminals (not shown), and relay signals transmitted between the base station and the mobile stations. The remote antenna systems 52-1 to 52-n are provided at predetermined locations far apart from the base station 50. The remote antenna systems 52-1 to 52-n include high-gain antennas 54-1 to 54-n, delay units 56-1 to 56-n, and remote antennas 58-1 to 58-n. 
A signal transmitted from the base station 50 is received and amplified by the high-gain antennas 54-1 to 54-n, and thereafter, are delayed by respective predetermined times in the delay units 56-1 to 56-n, and are transmitted from the remote antennas 58-1 to 58-n. In this system, the delay units 56-1 to 56-n provided in the remote antenna systems 52-1 to 52-n have different delay times that are multiples of a time τ that is slightly larger than the one-chip time of spread code. Thereby, for example, areas E58-1 to E58-5 which the remote antennas 58-1 to 58-5 cover, respectively, are formed as shown in FIG. 48B. In this case, by setting the arrival time difference of arriving waves at an area overlapping point at which signals from adjacent local antennas have substantially equal power and which are equidistant from the local antennas to be an appropriate value (in this case, about τ to 3τ), the path diversity by multi-station simultaneous transmission can be reliably obtained.
Also, Patent Document 2 describes a modulation scheme for a transmission method in which attention is paid to a symbol waveform (a phase waveform in a symbol). This scheme provides a symbol waveform having a convex-shape phase transition that is synchronous with a symbol length T, and obtains a detection output by delay (differential) detection. By this scheme, a situation in which a detection output is lost due to multipath can be avoided, and the path diversity effect is obtained, so that transmission characteristics can be improved. This improvement effect is theoretically achieved when the delay amount τ of delayed wave is within a predetermined range (0<τ<T).
FIG. 49 is a schematic diagram showing the phase transition of a symbol waveform described in Patent Document 2. In FIG. 49, in this phase transition, a phase is parabolically changed based on a function represented by:φ(t)=(4φMAX/T2)·1·(T−t);(0<t<T)  (1)where a transition width when the time length of one symbol (symbol length) is T is limited by a maximum phase transition amount φMAX.
FIG. 50 is a diagram showing a configuration of a transmission signal generating circuit 700 described in Patent Document 2. As shown in FIG. 50, the transmission signal generating circuit 700 comprises a differential encoding circuit 701, a waveform generating circuit 702, a quadrature modulator 704, and an oscillator 703. The transmission signal generating circuit 700 differentially encodes transmission data using the differential encoding circuit 701, modulates the resultant transmission data with a symbol waveform having convex-shape phase redundancy using the waveform generating circuit 702, and converts the resultant transmission data into a signal having a carrier frequency band using the quadrature modulator 704.
Next, a phase relationship between arriving signals when such a symbol waveform having convex-shape phase redundancy is used, will be described.
FIG. 51 is a schematic diagram showing a phase relationship between two arriving signals A and B when the symbol waveform having convex-shape phase redundancy is used. In FIG. 51, when a phase difference α is assumed to be 180 degrees, then if a delay occurs between arriving signals, the convex-shape phase transition allows an interval in which received waves remain without being canceled (points a and c in FIG. 51), though there is an interval in which received waves are canceled and lost in an effective interval (point b in FIG. 51). By processing the arriving signals A and B by a combination of delay (differential) detection and a low-pass filter, an effective detection output can be obtained. As a result, the path diversity effect is obtained and the transmission characteristics are improved.
FIG. 52 is a schematic diagram showing a configuration of a conventional wireless transmission system using transmission diversity by the modulation scheme described in Patent Document 2. As shown in FIG. 52, a delay unit 901 is provided between the transmission signal generating circuit 700, and first and second antennas 904 and 905 so as to provide a delay between signals to be transmitted from the first and second antennas 904 and 905. In this case, a delay amount is set so that the path diversity effect is satisfactorily exhibited before transmission is performed, so that transmission characteristics can be improved.
On the other hand, in recent years, a multi-hop system has been studied in which a plurality of wireless stations relay data to each other for wireless communication. FIG. 53 is a diagram showing a configuration of a conventional wireless transmission system described in Patent Document 3. In FIG. 53, the wireless transmission system comprises six wireless stations 17-1 to 17-6. FIG. 54 is a diagram schematically showing transmission timings of packets transmitted by the wireless stations of FIG. 53.
Initially, the wireless station 17-1 transmits a broadcast packet. The packet transmitted by the wireless station 17-1 can be received by the wireless stations 17-2 and 17-3 that are located in proximity to the wireless station 17-1. The wireless stations 17-2 and 17-3 wait for transmission from a timing when packet reception is completed to a predetermined transmission timing, and then simultaneously transmit packets.
Next, the packets transmitted by the wireless stations 17-2 and 17-3 can be received by the wireless stations 17-4 and 17-5. The wireless stations 17-4 and 17-5 also wait for transmission from a timing at which packet reception is completed to a predetermined transmission timing, and then simultaneously transmit packets. Thereafter, the wireless station 17-6 receives the packets transmitted by the wireless stations 17-4 and 17-5. Thus, in the multi-hop system of Patent Document 3, by using the multipath resistant OFDM, even when a plurality of wireless stations simultaneously transmit the same packet, interference does not occur. As compared to the case where the wireless stations 17-1 to 17-6 successively perform multi-hop transmission (in the stated order, one station for each time), a time required for broadcast packet transmission can be reduced, so that transmission efficiency can be improved.
Thus, according to the conventional wireless transmission system described in Patent Document 3, a plurality of wireless stations can perform efficient multi-hop transmission using a multipath resistant modulation/demodulation scheme.    Patent Document 1: Japanese Patent No. 3325890    Patent Document 2: Japanese Patent No. 2506748    Patent Document 3: Japanese Laid-Open Publication No. 2000-115181    Non-Patent Document 1: H. Takai, “BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform”, IEEE, Trans. Veh. Technol.), Vol. VT-42, November 1993, p 625-639    Non-Patent Document 2: S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T. Takeuchi, “A Power-efficient linear digital modulator and its application to an anti-multipath modulation PSK-RZ scheme”, Proceedings of IEEE Vehicular Technology Conference), June 1987, p 66-71